Carbon dioxide (CO2) is released into the atmosphere by the burning of wood, coal, oil and gas. It can also be released by volcanoes and livestock. The pre-industrial carbon dioxide level was around 278 ppm and had stayed fairly constant for several centuries. In the 20th century atmospheric carbon dioxide levels have increased from about 315 ppm in 1958 to 378 ppm at the end of 2004. Thus, since the beginning of the industrial revolution the concentration of carbon dioxide in the atmosphere has increased by around 36%. Carbon dioxide is a greenhouse gas, contributing to the increasing of the temperature of the earth. Carbon dioxide from the atmosphere is also absorbed by the oceans where it forms carbonic acid and increases the acidity of the water, impacting on populations of many life forms and threatening delicate ecosystems such as the coral reefs.
One method of reducing the amount of carbon dioxide in the atmosphere is to capture and store it as it is produced rather than release it into the atmosphere.
Of the various approaches to the capture and storage of carbon dioxide, the one that has gained the interest of governments and industries is storage of carbon dioxide in geological forms. The geological storage of carbon dioxide can be achieved in two ways: a) separate the carbon dioxide and pump it into empty or depleted oil wells (both terrestrial and oceanic oil wells can be used); or b) to convert it into calcium carbonate and dispose of it as landfill. One limitation of method a) is that there has to be a continuous monitoring of the oil well for possible leaks (especially for oceanic storage). The conversion to calcium carbonate for use in landfill is considered to provide a more reliable solution to the problem of carbon dioxide storage.
Calcium carbonate is a thermodynamically stable material and is abundantly found on the earth's surface. The calcium carbonate present on the earth is estimated to be a carbon reservoir equivalent to 150,000×1012 metric tons of carbon dioxide. Carbonates have been proven safe for long-term storage of carbon dioxide. An alternative to calcium carbonate is magnesium carbonate, which has similar properties. A combination of calcium and magnesium carbonate can also be used. The conversion of carbon dioxide into calcium carbonate (or other carbonates) is known as mineralisation. The rate limiting step in the mineralization of carbon dioxide is the hydration of carbon dioxide to form carbonate ions.
At present carbonic anhydrases (CAs) are seen as the most promising candidate for sequestering carbon dioxide. CAs catalyse the reversible hydration of carbon dioxide at mild pH values, with the fastest rates being observed for human CA II. The cost of extraction of enzymes limits their utility in the industrial context. They also only operate in a narrow pH range and so require the presence of a buffer. Additionally, enzymes can be unstable at elevated temperatures. There are some examples in the literature of carbonic anhydrase immobilised on nanoparticles being used for the hydration and capture of carbon dioxide (Vinoba et al., Langmuir, 2011, 27, 6227-6234; Vinoba et al., Journal of Molecular Catalysis B: Enzymatic, 75, 2012, 60-67). The nanoparticles were impregnated onto silica/alumina support.
Recently there have been a few reports on organometallic complexes being used for the reversible hydration of carbon dioxide. Holm et al (Inorg. Chem. 2011, 50, 100070-81; Proc. Nat. Acad. Sci., 2011, 108, 1222-7) have shown that nickel hydroxide complexes with 2,6-pyridinedicarboxamidate pincer ligands fix carbon dioxide very rapidly, but these processes are not catalytic. Organometallic compounds can be unstable to extreme conditions.
Accordingly, there remains a need for an improved method for the capture of carbon dioxide.